Nitrogen, phosphorus and potassium (NPK), which are required in large amounts for plants, are not adequately available in natural soils to support the sustained growth of plants. Therefore, these macronutrients (NPK) are needed to be applied externally through fertilizer. Water soluble conventional fertilizers typically result in a large amount of macronutrients being lost by leaching and evaporation. There is an increased interest in developing slow release fertilizers that release macronutrients to plants over time. Advantages of slow release fertilizers are improved efficiency and quality as the fertilizer is released over time thus providing sufficient quantities of macronutrients as required for higher crop yields. In addition, slow release fertilizers result in reduced environmental damage from leaching of macronutrients into water and emissions as gasses, compared to conventional water soluble fertilizers.
Macronutrients in fertilizers can be applied to the soil as a solid in the form of a powder or pellets or as a spray. The uptake of macronutrients by the plant needs to be compensated by their external application to the soil periodically. Nitrogen is a key macronutrient source in agriculture particularly for economic crops such as tea. Large amount of fertilizer is applied to the soil of the tea plant to improve the quality and the yield of the leaves produced. For example, a study in Japan (Yamada et al., Journal of Water and Environmental Technology, 7, 4, 331-340, 2009) reported that of the large amount of nitrogen fertilizer applied to tea, only 12% of the nitrogen input was taken up by the plant and the rest discharged to the environment. Therefore, one of the unsolved problems of fertilizer application is, in relation to the amounts of nitrogen applied to soil, the low Nitrogen Use Efficiency (NUE) by crops. This is because an excessive amount of nitrogen, up to 70%, is lost in conventional fertilizers due to leaching, emissions, and long-term incorporation by soil microorganisms. As such, supplying nitrogen macronutrient is critical in preventing the decline of productivity and profitability due to degradation and aging of tea plants (Kamau et al., Field Crops Research 1, 108, 60-70, 2008). Attempts to increase the NUE have so far has met with little success.
U.S. Pat. No. 6,261,997 B1 to Rubin et al., discloses slow release of pesticides adsorbed on organically modified clay to prevent leaching in underground and surface water. U.S. Pat. No. 4,219,349 to Bardsley, discloses compositions of calcined clay granules and solution or suspension containing micronutrients (Fe, Zn, Mn, Cu, B, Mo, Cl and S). U.S. Pat. No. 4,849,006 to Milburn et al., discloses a controlled release composition comprising of an organic, biologically active material absorbed on an organically modified clay. U.S. Pat. No. 6,821,928 B2 to Ruskin, discloses a method to reduce the rate of diffusion of slow release materials through polymers and a process for making drip irrigation devices with long term control of root growth. It further, discloses bioactive material such as herbicide that is intercalated into nanoclays to protect against root intrusion in drip irrigation applications. U.S. Pat. No. 3,902,886 to Banin et al., discloses clay attached micronutrients to provide micronutrients to plants. US2009/0169524 A1 to Kalpana et al., discloses biopolymer based nanocomposites of chitosan, montmorillonite (MMT) and hydroxyl apatite for bone growth in medical applications.
Solutions are needed to provide slow release macronutrient formulations for plant growth applications.